How to post an associative array to PHP that requires no jquery

How to post an associative array to PHP that requires no jquery
For example
foo[key1]=value1;
Foo[key2]=varlue2;
How can I POST it to PHP and fetch the associative array from $_post[' foo ').
Online check, are used juery, but I used an AJAX pool, only used the XMLHttpRequest object implementation,
Without jquery, if you use jquery instead, make a big change, so ask yourself how to do this without jquery.

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You always have to construct a parameter string.
Foo[key1]=value1&foo[key2]=varlue2&, .....
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Forms cannot transfer groups, only strings are constructed

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